The target frameworks are currently hard coded within the NuGet source code. Whilst some of the target frameworks are documented on the NuGet website there are many which are not documented there. So as Jeff R already mentioned the best way to find the target frameworks is currently to look at the NuGet source code.
NuGet also has short identifiers for some of these frameworks (e.g. wp = WindowsPhone).
The runtime identifiers defined by Microsoft are available in the Microsoft.NETCore.Platforms NuGet package. Inside this NuGet package is a runtime.json file with the runtime identifiers:
{
"runtimes": {
"base": {
},
"any": {
"#import": [ "base" ]
},
"win": {
"#import": [ "any" ]
},
"win-x86": {
"#import": [ "win" ]
},
"win-x64": {
"#import": [ "win" ]
},
"win7": {
"#import": [ "win" ]
},
"win7-x86": {
"#import": [ "win7", "win-x86" ]
},
"win7-x64": {
"#import": [ "win7", "win-x64" ]
},
"win8": {
"#import": [ "win7" ]
},
"win8-x86": {
"#import": [ "win8", "win7-x86" ]
},
"win8-x64": {
"#import": [ "win8", "win7-x64" ]
},
"win8-arm": {
"#import": [ "win8" ]
},
"win81": {
"#import": [ "win8" ]
},
"win81-x86": {
"#import": [ "win81", "win8-x86" ]
},
"win81-x64": {
"#import": [ "win81", "win8-x64" ]
},
"win81-arm": {
"#import": [ "win81", "win8-arm" ]
},
"win10": {
"#import": [ "win81" ]
},
"win10-x86": {
"#import": [ "win10", "win81-x86" ]
},
"win10-x64": {
"#import": [ "win10", "win81-x64" ]
},
"win10-arm": {
"#import": [ "win10", "win81-arm" ]
},
"aot": {
"#import": [ "any" ]
},
"win-aot": {
"#import": [ "win", "aot" ]
},
"win-x86-aot": {
"#import": [ "win-aot", "win-x86" ]
},
"win-x64-aot": {
"#import": [ "win-aot", "win-x64" ]
},
"win7-aot": {
"#import": [ "win-aot", "win7" ]
},
"win7-x86-aot": {
"#import": [ "win7-aot", "win7-x86" ]
},
"win7-x64-aot": {
"#import": [ "win7-aot", "win7-x64" ]
},
"win8-aot": {
"#import": [ "win8", "win7-aot" ]
},
"win8-x86-aot": {
"#import": [ "win8-aot", "win8-x86", "win7-x86-aot" ]
},
"win8-x64-aot": {
"#import": [ "win8-aot", "win8-x64", "win7-x64-aot" ]
},
"win8-arm-aot": {
"#import": [ "win8-aot", "win8-arm" ]
},
"win81-aot": {
"#import": [ "win81", "win8-aot" ]
},
"win81-x86-aot": {
"#import": [ "win81-aot", "win81-x86", "win8-x86-aot" ]
},
"win81-x64-aot": {
"#import": [ "win81-aot", "win81-x64", "win8-x64-aot" ]
},
"win81-arm-aot": {
"#import": [ "win81-aot", "win81-arm", "win8-arm-aot" ]
},
"win10-aot": {
"#import": [ "win10", "win81-aot" ]
},
"win10-x86-aot": {
"#import": [ "win10-aot", "win10-x86", "win81-x86-aot" ]
},
"win10-x64-aot": {
"#import": [ "win10-aot", "win10-x64", "win81-x64-aot" ]
},
"win10-arm-aot": {
"#import": [ "win10-aot", "win10-arm", "win81-arm-aot" ]
}
}
}